- tried it out in my work project, and realized the "op" and "context" namespaces

- tried it out in my work project, and realized the "op" and "context" namespaces
need to be there fully and in particular "context" needs to be
a proxy object, as env.py may have dependencies which live beyond the
scope of the migration script.   Will have to try to make
these proxies as straightforward as possible.
- more architecture docs